Expand Documentation on Use of (optional) `tenant` Label

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## Describe the Feature

The README currently does not explain the optional `tenant` label and how `stacks.name_pattern` can be configured to either use it or not use it.

## Expected Behavior

The README should clearly and concisely explain how to configure atmos to use the optional `tenant` label, and how to write create the stack YAML configs accordingly.

## Use Case

Users attempting to use `atmos.yaml` in its default form will be forced to use the `tenant` label, and no documentation indicates that this is an optional label and how to configure `atmos` to not use this label.

## Describe Ideal Solution

Expand the documentation as described in _Expected Behaviour_.

## Alternatives Considered

N/A

## Additional Context

https://sweetops.slack.com/archives/CQCDCLA1M/p1643728355414279